Do you have other coral in the tank besides the zoas? How long, and how are they doing? Have you changed your light bulbs, if they are a year old? T5HO's should be changed every 6-12 months. The light you describe, if the bulbs are good, should be sufficient for zoas.
Would you characterize where the new zoas are in the tank as low flow, medium flow, or high flow? How about the same for lighting?
Understanding your calcium, alkalinity, and to a lesser extent mag levels are important to knowing if it is a water quality issue.